This invention relates to a CRT (cathode-ray tube) having an improved implosion-protection system.
One form of CRT comprises an evacuated envelope including a glass faceplate panel having a viewing window and a rearwardly-extending peripheral flange or sidewall. The extended end of the sidewall is sealed to the large opening of a glass funnel. Atmospheric pressure pressing against the external surface of the window may exert forces totaling several tons against the window. Should the window shatter (implode), these forces drive fragments into the tube, which fragments may bounce back through the shattered window and may cause injury to persons nearby. Also, when the window shatters, fragments of the funnel may also be driven through the shattered window. In this art, the amount of glass in fragments that are driven or thrown toward the viewer is called the throw of glass. To add strength and stability to the structure, most CRT windows are arched or domed, which has the effect of reducing the amount of glass fragments that are driven by the implosion.
It has been proposed to render a CRT more resistant to implosion and to reduce the throw of glass during the implosion by encircling the sidewalls of the panel with a metal and/or plastic reinforcing structure, which will hold the sidewall in place should fracture occur, until the tube is substantially devacuated. In some forms, the reinforcing structure is attached to the tube with an adhesive, and may or may not include a tensioned metal band encircling the sidewalls.
More recently, it has been proposed to use a coating of polyurethane on external portions of the envelope. Such a coating has been proposed for use by itself alone or in nonoverlapping relationship with metal reinforcing structures.
There are some CRT types in which the window of the tube is substantially flat where the prior reinforcing structures have not proved to be adequate. In such flat-faced tubes, described, for example, in U.S. Pat. No. 3,416,026 to I. Niwa and No. 3,837,829 to P. Lebel, there are far less strength and far less stability in the window, with the result that a much smaller blow causes the window to shatter and, also, when shattering occurs, the entire window fragments in the implosion. It is not possible to prevent the shattering of a substantially flat CRT window or to reduce the amount of driven glass with an implosion-protection system. However, the throw of glass can be reduced by the novel combination so that the CRT is safe to use.